1st part
We have,
Worker W1=5
Weed a field in days=D1=12days
If,
Take worker W2=6
Then, Days D2=?
Then, we know that,
W1D1=W2D2
5×12=6×D2
D2=5×126
D2=10days
2nd Part,
If workerW3=15
Then,
Days D2=?
Then, we know that,
W1D1=W3D3
5×12=15×D3
D3=5×1215
D3=4days